NPDP Dam Dictionary

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Term AKA Definition
Select material Backfill materials specially selected and segregated from excavated materials.
Semipervious zone See transition zone.
Sensitive species Species not yet officially listed but undergoing status review for listing on the U.S. Fish and Wildlife Service's (FWS) official threatened and endangered list; species whose populations are small and widely dispersed or restricted to a few localities; and species whose numbers are declining so rapidly that official listing may be necessary.
Separation deposit Sand deposit located at the upstream end of a recirculation zone, where downstream flow becomes separated from the channel bank.
Service spillway A structure located on or adjacent to a storage or detention dam over or through which surplus or floodwaters which cannot be contained in the allotted storage space are passed, and at diversion dams to bypass flows exceeding those which are turned into the diversion system. Included as part of the spillway would be the intake and/or control structure, discharge channel, terminal structure, and entrance and outlet channels.
Settlement The sinking of land surfaces because of subsurface compaction, usually occuring when moisture added deliberately or by nature, causes a reduction in void volumes.
Shaft house House at top of the access shaft to gate chamber, which secures access to the gates.
Shear A structural break where differential movement has occurred along a surface or zone of failure.
Shear strength The maximum resistance of a soil or rock to shearing stresses.
Shear zone An area where the rock mass has moved along the plane of contact which often becomes a channel for groundwater.
Sheepsfoot roller A tamping roller having lugs with feet extending at their outer tips.
Shell See shoulder.
Shelterbelt A natural or planned barrier of trees or shrubs to reduce erosion and provide shelter from winds or storms.
Short-throated flumes Short-throated flumes are considered short because they control flow in a region that produces curvilinear flow. While they may be termed shortthroated, the overall specified length of the finished structure, including transitions, may be relatively long. The Parshall flume is the main example of this kind of flume. These flumes would require detailed accurate and accurate knowledge of the individual streamline curvatures for calculated ratings which is usually considered impractical. Thus short-throated flumes are determined empirically by comparison with other more precise and accurate water measuring systems.
Shoulder Shell The upstream and downstream parts of the cross section of an embankment dam on each side of the core or core wall. Hence the expression upstream shoulder or downstream shoulder.
Shrinkage index SI The numerical difference between the plastic and shrinkage limits.
Shrinkage limit SL The maximum water content at which a reduction in water content will not cause a decrease in volume of the soil mass.
Shrinkage ratio R The ratio of a given volume change, expressed as a percentage of the dry volume, to the corresponding change in moisture content above the shrinkage limit, expressed as a percentage of the weight of the oven-dried soil.
Significant hazard A downstream hazard classification for dams in which 1-6 lives are in jeopardy and appreciable economic loss (rural area with notable agriculture, industry, or worksites, or outstanding natural resources) would occur as a result of failure of the dam. This classification also applies to structures other than dams.
Sill A submerged structure across a river to control the water level upstream. The crest of a spillway. The horizontal gate seating, made of wood, stone, concrete, or metal at the invert of any opening or gap in a structure. Hence, the expressions: gate sill, stoplog sill.
Silt The fine-grained portion of soil that is nonplastic or very slightly plastic and that exhibits little or no strength when air dry. Nonplastic soil which passes a No. 200 United States Standard sieve.
Simulid Group of two-winged flying insects who live their larval stage underwater and emerge to fly about as adults.
Sink Depression in the land surface, especially one having a central playa or saline lake with no outlet.
Sinkhole A steep-sided depression formed when removal of subsurface embankment or foundation material causes overlying material to collapse into the resulting void.
Sinuosity The ratio of the length of a river's thalweg to the length of the valley proper. A measure of a river's meandering. Rivers with a sinuosity less than 1.5 are usually considered straight.
Siphon A pipe connecting two canals.
Site In archeology, any location of past human activity.
Slaking The process of breaking up or sloughing when an indurated soil is immersed in water.
Slide gate A steel gate that upon opening or closing slides on its bearings in edge guide slots.
Slope An inclined surface usually defined by the ratio of the horizontal distance to the vertical distance, i.e. 2:1 (2 horizontal units to 1 vertical unit). Change in elevation per unit of horizontal distance (i.e. 0.50). Side of a hill or a mountain. The inclined face of a cut, canal, or embankment. Inclination from the horizontal. Expressed in percent when the slope is gentle; in this case also termed gradient.